This is an
electronic reprint of my material that appeared in the history chapter of MIXED
GAS DIVING published by Watersport.This material is copyrighted and all rights retained by the author. This
article is made available as a service to the diving community by the author and
may be distributed for any non-commercial or Not-For-Profit use.

It was called "A Study In Controlled Paranoia." It was the
deepest ever dive made by a free-swimming scuba diver.On April 5, 1988, Sheck Exley added
another exploration to his personal log. This event was a milestone in diving
history; it was not a typical sport dive. Sheck plunged to a depth of 780 feet
within the Mexican cave system known as Nachimento Del Rio Mante.

The descent took 24 minutes; the decompression time was longer
than 10 hours. The decompression times and gas mix utilized were controlled by a
computer generated protocol developed by Bill Hamilton and Dave Kenyon known as
the DECAP (Decompression Computation and Analysis Program). Since there was no
previous experience with free-swimming "sport" divers at 700 feet, the tables
were considered experimental. In addition to the four cylinders that Sheck
carried, the dive protocol called for 16 staged bottles at depths between 30 and
270 feet, containing 11 different gas mixes, with 52 separate decompression
stops beginning at 520 feet. The dive ended at the surface with breathing pure
O2 for 30 minutes.

The dive was the essence of the true explorer: a highly
skilled individual testing theoretical decompression protocols while venturing
into areas previously unknown to mankind. If this had been a pilot's adventure,
Hollywood would already have released the movie (starring Errol Flynn), and
Sheck would be battling Teenage Mutant Ninja Turtles for prime time ratings. As
it was, after finishing the dive, Sheck and his team loaded the van and drove
home... just another day of diving.

The cave diving community has consistently been on the cutting
edge of technology. Their continual drive to go deeper and further than anyone
before has given rise to much of what is known in the recreational dive
community about the use of gas mixes other than air for deeper diving. Sheck's
dive was not an isolated event, but rather the pinnacle of a series of carefully
planned advances in knowledge about deeper diving.The advancement of knowledge has not
been without its price. Along the way, pioneers and adventurers have died while
seeking to dive deeper, extend bottom time and cover more distance.We, who sport dive today, owe our
"recreating" to those who have gone before.

HELIUM

In 1919,
Professor Elihu Thompson, an electronics engineer and inventor, speculated that
nitrogen narcosis could be avoided if the oxygen in the breathing mix were
diluted with a gas other than nitrogen. Thompson had previously established a
record as an innovator with seven hundred patents including electric welding,
the centrifugal cream separator and street arc lights. His business had earlier
merged with Thomas Edison's company to form General Electric. He suggested that
helium would be a suitable gas for deep diving without narcosis.Since at the time, the price of helium
was over $2500 per cubic foot, the suggestion was viewed as an economic
impossibility. About this time C.J. Cooke applied for a patent on the use of
Helium as a breathing gas mix. Additionally a series of experimental dives was
begun on the U.S.S. Falcon, which included at least one dive to 150 feet on a
heliox mix. Later, the discovery of helium in four Texas natural gas wells gave
the United States an exclusive monopoly on the world's supply of helium. Its
abundance dropped the price of helium to a few cents per cubic
foot.

Thompson convinced the Bureau of Mines, which controlled the
world's supply of helium (and was desperately seeking some use for this gas),
and the US Navy to begin examining the potential for deep diving using helium
and oxygen as a breathing gas mix. By 1925 a lab had been established in
Pittsburgh and lab animals were doing simulated dives in a chamber using
helium-oxygen mixes.This work
established that animals breathing an 80 % helium / 20% oxygen mix could be
decompressed at 1/6 the decompression time of an air breathing animal.Later, humans subjects breathing 80%
helium / 20% oxygen were found to have no apparent problems with heliox
decompression schedules that were 1/4 the time required for air breathing dives.
More importantly, however, was the ability for humans to function "clear-headed"
at depths where air-breathing divers were incapacitated by nitrogen narcosis.
Thompson's second major contribution to deep diving followed this early effort.
He suggested that since the helium was not consumed during the dive, it could be
conserved by use of a re-circulating system for the diluent gas. His idea would
ultimately prove sound; it needed only the development of high efficiency
absorbents to remove the carbon dioxide generated from human metabolism so that
the exhaled gas could be re-circulated.

Divers commented on the ease of breathing helium, but noticed
that they always felt chilled while breathing heliox mixes. The change in voice
characteristics often made communication at depth difficult. It was apparent
that the narcosis free advantage of helium breathing would create problems as
well as solutions. It was obvious that much work still needed to be
done.

Funding for deep diving training was very difficult to obtain
in the post World War I economy. Although the US Navy Experimental Diving School
in Newport, Rhode Island had successfully trained divers that had salvaged the
sunken submarine F-4 in 304 feet of water off Amala Bay near Honolulu, Hawaii in
1915, Congress could not be convinced to provide the funds for continuing the US
Navy Experimental Diving School in Newport.This school had, before the First World
War, conducted over three hundred test dives on air to depths of 258 feet. This
work was the foundation for the first diving manual published in 1924 by the
Bureau of Construction and Repair.

On September 25, 1925, the submarine S-51 sank after a
collision with the S.S. City of Rome near Block Island in 132 feet of
water.Since the Navy had not had
the funds to maintain their deep diving training program and associated
submarine rescue proficiency, the salvage of this vessel took many months.
(There simply were not enough divers qualified to dive below 90 feet. Bad
weather and extreme cold also hampered the salvage effort.) Despite the disaster
and modest public inquiry, funding was still denied. However, the public began
to slowly build interest in improving the Navy's ability to function at deeper
depths.

In 1927 another sub, the S-4 sank in 102 feet of water with
loss of all forty men. One sad aspect of this disaster is that six men survived
the sinking and their taps on the hull could be heard by divers working to raise
the vessel. For a short time, two-way communication via rapping on the hull
existed until, eventually, the taps from inside the hull ceased.The salvage operation lasted for over
three months.An indignant public
began to demand an improved Navy capability. As a result, the Navy established
an Experimental Diving School in Washington, D.C. The two primary missions were
to develop diving techniques to limit the effects of nitrogen narcosis and to
develop rapid, effective methods for the rescue of crew trapped inside sunken
submarines.

One significant early achievement of this Navy research unit
was the development of the McCann rescue bell. This was a diving bell or chamber
that could be lowered on cable to mate with the escape hatch on a submarine.
Once attached, the water in the chamber between the bell and the submarine could
be blown away by the use of compressed air. This then allowed trapped
submariners to open their hatch and move into the rescue bell. The bell hatch
could be closed and the chamber pulled back to the surface by winch. This bell
would prove itself as this rescue device saved many trapped
submariners.

In the late 1930's, an intern at Milwaukee County General
Hospital, Edgar End, investigated the use of helium and oxygen as a breathing
gas mix. His friend, Max Gene Nohl, an MIT graduate student, had developed with
the assistance of John Craig (later of "Danger Is My Business" television fame),
a new type of diving helmet.This
helmet was part of a self-contained helium-oxygen system with on-board scrubbing
of carbon dioxide that had been developed with the goal of photographing the
Lusitania in 312 feet of water. During their pre-expedition trials, they were
able to work at 312 feet for up to two hours with only three brief stops on
their thirty minutes ascent to the surface.When the expedition to photograph the
Lusitania was abandoned, it was decided to attempt a world depth record. On a
cold December day in 1937 the self-contained heliox helmet was tested in near-by
Lake Michigan. Max Nohl set the world's depth record at 420 feet. The dive was
conducted using a suit that was definitely not conventional.The diver wore a fashionable rubber
coverall with Eskimo-like mukluks boots and a helmet that was described as
looking like a lighthouse top with windows on all sides. The diver wore two
self-contained tanks of breathing gas.

About the same time, the US Navy research program began
gathering significant momentum under the guidance of Behneke and Yarbrough.They successfully completed a simulated
chamber dive to 500 feet using heliox. It is interesting to note that during the
Navy tank dive, the diver did not know his depth. When asked about his depth,
the diver replied, "It feels like a hundred feet." During his decompression, the
diver was told his actual maximum depth.

This success prompted the Navy to increase investigative
efforts in the use of helium in diving gas mixes. By 1939 experimental research
was a reality when the submarine, U.S.S. Squalus, sank off the Isles of Shoals
in 243 feet of cold North Atlantic water.Since the submarine had been quickly located, there was a frantic effort
to rescue the men trapped on board. A downhaul cable (for the McCann Rescue
Bell) had parted and a diver on compressed air had been unable, due to the
crippling effects of nitrogen narcosis, to repair the cable. It was decided to
try the new helium-oxygen mixture.A diver on heliox was successful.The McCann rescue bell made four trips in twelve hours to the sunken
submarine and 33 men were successfully rescued. The submarine was then salvaged.
The US Navy conducted over 100 dives without injury in the rescue/salvage
effort. The U.S.S. Squalus, renamed the Sailfish, served in World War II.The rescue of thirty-three men and the
successful salvage of the Squalus demonstrated to the US Navy and the American
public that heliox was a viable protocol for deep diving operations. The
successful rescue of these trapped men and the subsequent salvage of the sunken
vessel are considered to be two of the most significant accomplishments in the
history of marine life saving and salvage operations. The completion of the
Squalus salvage and the appearance of war clouds on the horizon prompted
Congress to increase the funding for US Navy deep diving training and
research.Incidentally, Congress,
fearing both the ingenuity of German research under the political control of
Adolph Hitler and the use of helium in lighter-than-air dirigibles, prohibited
the export of helium.For the next
twenty years the US Navy, with the world's sole supply of helium, was the
primary user/investigator of heliox as a breathing gas mix.

Jack Browne, devised a triangular lightweight mask and tested
his system in 1946. Diving in a pressurized tank, with decompression guidance
from End and Behnke, he did a simulated dive of 550 feet.

Following the Second World War, the British, using helium that
had been obtained with the approval of the US Secretary of Interior, Secretary
of Defense and the US President, began experimenting with heliox mixtures. In
some of their first dives, the divers developed extreme claustrophobia during
diving and in screaming fits demanded to be hauled out of the water. Although
the physiologists were convinced that the oxygen concentrations were too high
and thus the source of the problems, the divers blamed the helium. It was
referred to as "Yankee gas" or "Stuka juice." (The Stuka was a German
dive-bomber used during WW II.)

The British diving experiments were conducted from the vessel
Reclaim. This vessel anchored near a vertical wall in Loch Fyne that bottomed
out at 540 feet. Since the Olympics were being held in London, Captain Shelford
devised a "diver's depth thermometer" and sent divers on eighteen dives, each
deeper than the previous. The depth's reached were recorded on the
"thermometer." A Diver's torch was created and each new depth reached was
rewarded with the diver receiving the "Olympic torch." The diver would accept
the torch and run a victory lap around the vessel while wearing diving
boots.Using nearly all of the
remaining British supply of helium, Diver First Class Wilfred Bollard took 7 1/2
minutes to descend to a new world record depth of 540 feet. After 5 minutes on
the bottom the diver took 8 hours and twenty-six minutes to ascend (descent time
was increased because of a three hour treatment for the bends; the diver
suffered elbow pain when transferring from the Davis submersible decompression
chamber (which he had entered at 190 feet) to the main decompression chamber at
30 feet.)After leaving the
decompression chamber he received the "torch" from the "hand" of a Neufeldt and
Kuhnke armored diving suit. Since this state-of-the-art suit was rated to 500
feet, Bollard scratched "OUT OF DATE" on the suit and then did his victory
lap!

In
1968 Heliox entered the world of "amateur" cave diving. A group of
BSAC divers primarily from Rhodesia and South Africa, spent more than year
assembling gear, support equipment and personnel to dive the Silent Pool in
Sinoia, Rhodesia. Divers Roly Nyman, lan Robertson, John van der
Walt and Danny van der Walt dove
to 340 fff on Tri-Mix (Nitrogen 41.6%: Helium 40 %: 18.4% Oxygen)
while exploring the muddy bottom of this famous sink-hole.

In 1970, ex-Navy diver Tom
Mount brought heliox into American scientific diving at the University of Miami
Rosenteil School of Marine Science. About the same time, Hal Watts, in
Florida, experimented with heliox for deep cave operations.

Dale Sweet, in
1980, used heliox to reach 360 feet in the cave Die Polder #2. Although six
months later, Sheck Exeley made the same dive on compressed air, the cave diving
community was beginning to notice the existence of mixed gases for deep
explorations. It would take three years, but in 1981, a German cave diver,
Jochen Hasenmayer, descended into the French Vaucluse to reach a depth of 476
feet using heliox.This was a new
world record for a surface-to-surface scuba dive. In 1983, Jochen made another
heliox dive. This dive was to 685 feet and another world
record.

One of the most well known uses of mixed gas within the cave
diving community was the 1987 exploration of the Wakulla Springs Cave system.
Using twelve divers, the team penetrated the cave more than 4000 feet at depths
near 300 feet. The dive operation decompression profiles were controlled by a
new computer analysis protocol developed by Bill Hamilton and Dave Kenyon. Much
of the success of the exploration was due to the use of mixed gas under the new
decompression guidelines. Now, with reasonable decompression predictions, the
depth records would move downward and more cave systems would be systematically
explored.

Sheck
Exley, a living cave diving legend, personally took up
the challenge to extend the depth range of cave divers. After practice heliox
dives in Florida springs, Sheck went to Mexico and made "tune-up" cave dives of
515 and 660 feet. Later, On April 1, 1988, he extended the world record for free
swimming scuba divers to 780 feet.

HYDROGEN

The first
recorded use of hydrogen as a breathing mix was in 1789. Lavoisier (The Father
of Modern Chemistry) and Sequin exposed guinea pigs to mixtures of hydrogen and
oxygen (Hydrox). They observed that the animal's oxygen consumption appeared to
be similar in hydrogen/oxygen as in nitrogen/oxygen. Prior to WW II, a Russian
scientist, Lazarev, subjected a single mouse to elevated pressures of hydrogen
and oxygen. However, the use of hydrogen as a breathing gas for diving
operations is generally associated with the Swedish Engineer, Arne Zetterstrom. Hydrogen is a desirable breathing gas component because it is
the lightest element known. This means it is the least dense at depth and
breathing resistance is minimal at extreme pressures. The major problem with hydrogen-oxygen mixtures
is the potential for explosion. Although the concentration of oxygen needed for
combustion of oxygen-hydrogen mixes varies a bit with pressure, a general rule
of thumb is that hydrogen-oxygen mixes above 5 % O2 are
at-risk.. So,
to avoid nasty fires and explosions, hydrogen is only considered as a breathing
gas component at pressures where a less-than 5% oxygen concentration in the
breathing gas mix gives a partial pressure of oxygen great enough to sustain
life.

Perhaps the most common example of a
hydrogen-fire related disaster is the destruction of the Hindenburg dirigible. The nearly 900
foot long airship burned on landing near Lakehurst, NJ in the late 1930's
after a successful trans-Atlantic crossing. While there is no doubt that the
hydrogen burned and its flames consumed the dirigible, the most likely cause of
the fire was not a hydrogen-oxygen problem. Recent analysis suggests a
slightly more different scenario. The airship was coated with a metal-particle
coating (that's how it got its classic silver sheen.) It is now believed that
this material acquired a substantial electrical charge because of near-by
thunder storm activity. The landing officials on-site requested, because of the
weather, that the vessel drop its landing lines from a high altitude. It is
believed that in-doing so, as soon as the lines hit the ground, the vessel
"grounded-out" and a substantial electrical charge was passed. The
electrical current carried by the metallic particles in the skin coating created
enough sparks to ignite the hydrogen and once ignited, the hydrogen burned with
enough intensity to destroy the once greatest-ever airship. This incident will
always remind us of the potential for disaster that is present whenever oxygen
and hydrogen are used in proximity..

In 1944 Arne Zetterstrom discovered a way to breach the
transition between compressed air and Hydrox without risking explosion. The
technique was to descend to 100 feet and switch to a 4% oxygen / 96 % nitrogen
mixture. After breathing this mix for sufficient time to allow the oxygen
concentration in the lungs to drop below the "explosion threshold," the diver
switched to Hydrox and continued descent. On ascent, the diver again used the
Nitrox (4% O2 / 96% N2) as a transition between Hydrox and air. Using
this technique, he descended to 363 feet. At that depth, the alteration in voice
characteristics, coupled with excitement, made communication impossible and
additional dives used a telegraph key.

On August 7, 1945 Arne intended to demonstrate the usefulness
of his technique for assisting in submarine rescue efforts by establishing a new
world record. It is believed that he dove on a support platform suspended from
the stern of the vessel. Due to intense current, a line was rigged from the bow
of the vessel to the support platform to maintain vertical orientation. He
dropped rapidly to 100 feet on compressed air and made the Nitrox transition to
Hydrox. Then, tapping telegraph messages during the

entire descent, he dropped to a record setting 528 feet. Since
no Hydrox tables existed at the time, Arne used his own calculations for the
ascent. His first staged decompression stop was intended to be 165 feet. The bow
winch stopped at 165, but an enthusiastic, but misguided stern crew continued
raising him to the surface. There was no gas transition and he ascended while
breathing 4% oxygen/hydrogen with no decompression stops. He died shortly after
reaching the surface. The death certificate stated that death was from "acute
lack of oxygen and caissons disease of a violent nature." Although his death was
totally unrelated to the use of hydrogen or his transition technique, (but by
what was termed "an unpardonable mistake"), research on this gas was
discontinued for many years.

During the mid 1960's research into the use of hydrogen in
breathing gases resumed with animals breathing Hydrox for up to 24 hours at 70
Ata. One interesting aspect of the animal research was the suggestion that
hydrogen reduced the HPNS (high pressure nervous syndrome) often observed with
helium based gas mixes on deep dives. Ultimately animals would be taken to 3500
feet on hydrox.

By 1967 there were two successful human chamber dives using
hydrogen as a breathing mix to 7 Ata for 10 and then 20 minutes. More
experimentation was resumed during the early 1970's to begin to develop
realistic tissue saturation times for the future development of Hydrox
decompression tables .In 1974 the US Navy initiated a series of dives termed
HYDROX to further evaluate problems associated with switching from Hydrox to
other gases.

In 1983 COMEX, the French deep diving concern (perhaps more
famous in the US as the company providing the submersible used in the recovery
of artifacts from the Titanic) began a series of dives to investigate the
narcotic potential of hydrogen. Divers including H.G. DeLauze, President of
COMEX, descended in open sea to approximately 300 feet for five minutes. The
divers could not perceive a difference between Hydox and Heliox at that
depth.Chamber dives to 300 m (984
ft) demonstrated that hydrogen possessed a narcotic effect different from
nitrogen. Hydrogen narcosis (the "hydrogen effect") had a tendency to be more
psychotropic – i.e. more like LSD, while nitrogen narcosis had an effect similar
to alcohol. This deeper work suggested that Hydrox as a binary gas mix would not
be too useful at depths below about 500 feet.

MIXMASTERING

A young mathematician and Zurich engineering school
instructor, Hannes Keller, saw his first aqualung while vacationing in Greece in
1958. After talking with local divers and with no previous experience in diving
he was said to have proclaimed, "Diving techniques were thirty years behind the
times." He decided that his life's work would be to improve diving technology. A
voracious reader, he soon had read much of what was then available on deep
diving technology. He convinced a cardiac specialist, Albert Buhlmann to join in
his efforts. Together they developed a technique for utilizing nine different
gas mixes at various depths and proposed that deep diving was possible by
varying the proportions of the gases at different depths.

Although no money was available for computer time, Keller
convinced IBM to give him four hours of computer time to do the necessary
calculations for developing tables based on the hypothesis of multiple mixes for
different depths. The result was four hundred different tables for depths up to
1312 feet. Next, he went to Jacques Cousteau and with his assistance was able to
have access to the French diving chamber that was operated by the French Navy
Group for Undersea Research and Development at Toulon. There, Cousteau and a
team of Swiss scientists watched Keller attain a pressure of 630 feet and ascend
without illness or apparent difficulty. When news of his success circulated
within the international diving fraternity, this community assumed that Keller
was some sort of physiological freak with extraordinary tolerances for breathing
exotic gas mixes at depth.

Keller went to the US to try to convince the US Navy to
finance further work. Although many in the US scientific diving community were
intrigued by Keller's claims, funding was denied. (Keller insisted that he would
retain all commercial rights and that his gas mixtures remained a secret.)
Keller then returned to France.

Although the French could not fund his work, they did allow
him access to the chamber in Toulon. Here Keller did a simulated dive to 1000
feet using a commercially available sport diving Calypso single hose regulator.
This got the world's attention, but unfortunately helped promulgate the myth
that Keller was indeed some freak of nature that could survive events that would
kill mere mortals.

In an effort to convince the world of the validity of his
diving theories, in 1961 he established a diving barge on Lake Maggiore near the
Swiss-Italian border.Here, with
Kenneth MacLeish (a 40 year old Science editor for Life magazine with some deep
diving scuba experience) as a dive buddy, he attempted to prove that his success
was due to the nature of his theory, not some unusual physiology.Wearing rubber suits and woolen
underwear, the pair descended on an elevator to 725 feet of fresh water
(equivalent to 685 feet of sea water). After inscribing their initials on a
mermaid doll that they had brought with them for luck, they took one hour to
ascend to the surface. Apart from the cold, they suffered no apparent ill
effects with no symptoms of the bends.

This event convinced many that Keller's work should be taken
seriously. In 1962, Keller with the financial support of the US Navy, some
publishers and an oil company, descended with Peter Small (a journalist and
founder of the British Sub Aqua club) in a chamber designed by Keller, The
Atlantis, to a depth of 1000 feet off Catalina Island. On the bottom Keller left
the chamber and planted Swiss and American flags. The flags were very large and
Keller became entangled in them.After struggling to get free, he returned to the bell with only moments
remaining in his gas supply.In his
desperate reentry at depth, Keller's fin caught in the gasket that sealed the
chamber. Without a gas tight seal, breathing gas was lost. (The chamber had been
designed to maintain a pressure inside greater than outside. A fin compromised
the integrity of the seal and thus resultant seal loss meant that gas would
rapidly escape.) Surface observers watched the two men on television as they
lost consciousness.After the
chamber had reached free diving depth (200 feet), Richard Anderson and Chris
Whittaker, dove to the chamber and freed the fin.Only Anderson returned.On return to the surface Small was found
dead.Small died of an embolism;
the cause was attributed to lack of oxygen while he was unconscious. An inquest
chaired by John Craig found death to be accidental; the result of exploration of
the unknown coupled with human stress to complete the dive.The board concluded, "Keller's
experiment produced a significant scientific achievement." While the experiment
ended in tragedy, it did demonstrate the viability of using multiple gas mixes
at depth.

The trend has been to investigate and use multiple component
mixes for extended diving. It was found in 1965 that divers breathing heliox
mixtures at depths below 500 feet developed nervous tremors known as High
Pressure Nervous Sydrome (HPNS). A series of deep dives at the F.G. Hall
laboratory at Duke University under the direction of Peter Bennett found that
using small quantities of nitrogen in the heliox (termed tri-mix) helped
eliminate this problem. In their Atlantis dive series, three divers reached a
depth of 2132 feet breathing tri-mix.

It has been found that adding helium to hydrogen-oxygen mixes
(termed Hydreliox) helps to eliminate the "hydrogen effect" narcosis associated
with breathing only a hydrogen-oxygen mix. Theoretical limits of hydreliox are
currently placed at about 1750 feet.

NITROGEN

Henry
Fleuss, Master Diver for Siebe, Gorman & Co. of London, conducted the first
documented dive using oxygen-rich air. Breathing an estimated 50-60%
O2, Mr Fleuss spent an hour in a large tank. A week later, he used
his apparatus in open water. He was injured when tenders on the dive abruptly
pulled him to the surface. The dives were conducted in 1879. By 1912 Robert
Davis and Leonard Hill had devised a self-contained rigid diving helmet that
utilized a 50% oxygen-nitrogen mixtu

re. This apparatus was used under the
guidance of J.S. Haldane to a working limit of 100 fsw. Since the nitrogen
concentration was much less than air, this device demonstrated the
then-remarkable decompression advantage available from using an oxygen-enriched
air breathing mixture.

The first commercial use of nitrogen-oxygen mixes of other
than normal air concentration was the self-contained dress of the Westfalia
Machinenfabrik in Geisenkirchen, Germany.In 1912 they used their suit with a mixture of 45% and 55% O2
for depths to a 100 feet and a 30% O2 mix for diving to depths of 200
feet. This suit, or the Nitrox blend did not receive wide distribution. Based on
this work, in 1913 Draegerwerk produced a similar device that automatically
mixed nitrogen and oxygen supplies to produce a 60 percent O2 mix.
Some time before WW I, the Fleuss-Davis SCUBA unit appeared. This device
consisted of two 10 cubic foot tanks; one each for compressed air oxygen. The
gases were mixed in a manifold between the two tanks and the diver's mouthpiece.
The manufacturer claimed success of this unit to depths of 66
feet.

Between the two world wars Siebe Gorman & Co. introduced
the technique of using different concentrations of oxygen mixed with nitrogen.
It had been established that divers could not tolerate oxygen concentrations
greater than 2 Ata for extended periods of time without difficulty. The divers
affected by this incapacity and convulsions associated with high O2
concentrations invented a mythical monster, "Oxygen Pete," who was supposed to
lurk on the bottom of the sea waiting to molest unwary divers. Oxygen toxicity
hits during this time were referred to as "getting a Pete."

Perhaps the best-kept secret of WWII was the use of
oxygen-enriched air re-breathers by the British commandos defending Gibraltar.
Those attacking the British strong-hold were using 100% O2 re-breathers.The deeper maximum operating depth of
the British mixes (45-60% O2) was a distinctive underwater combat
advantage since opposing divers (using 100 % O2) would be at
“convulsive depths” while the British divers were still within their operating
parameters. A major component of the British strategy was to simply take the
opponent down until convulsions overwhelmed the enemy diver. This secret was so
well kept, that much of this was not even revealed to the US Navy until the
1950’s.

One interesting feature of the British combat protocols was
the definite association between CO2build-up and increased susceptibility to oxygen toxicity seizures.
Their orders forbade rapid swimming unless “demanded by enemy contact.”Much of our present knowledge of oxygen
enriched air mixes can be traced directly to the British research efforts on
oxygen and oxygen-enriched air breathing mixes conducted during
WWII.

Since it was known that increased oxygen (decreased nitrogen)
increased time available bottom time without decompression obligation in the 60
-100 foot range, a number of mixes were utilized primarily by the commercial
diving community during the period following WW II.

Workman developed decompression schedules for nitrogen-oxygen
and helium-oxygen diving and published these tables in 1965.

In the late seventies, the Canadian research institute DCIEM
was asked to develop a diving apparatus for the Canadian military to be used in
clearing mines. The ideal system would not disturb mine sensors that would
detect motion, magnetic fields, and/or sound. It was decided to utilize a
semi-closed system that would use a nitrogen-oxygen mix that would vary in
O2 concentration at depth to supply a constant p O2. The
constant pO2 is delivered via a pneumatic manifold, as opposed to an
electronic pO2 sensor controlled relay system. This system was made
available in the late 80's.

In 1978 NOAA formally established procedures for a standard
mixture of 68% N2 / 32% O2. It is known as NOAA Nitrox I. A second
standard mix containing 36% O2 is known as NOAA Nitrox II. Since that
time, a number of users, including the US Navy, commercial and academic diving
operations have successfully used Nitrox in operations shallower than 130 feet.
NOAA has developed a reasonable compact shipboard continuous gas mixing system
to supply Nitrox for diving operations.

During the last five years, approximately 28,000 logged dives
using Nitrox were surveyed. Although rigorous statistical analysis is not yet
complete, the trend is that Nitrox is a safe, easily handled mix when used by
properly trained divers. Two different agencies, ANDI and IAND, have been formed
to introduce this technology to the sport diving communities. The sanctioning of
Nitrox training by recreational training agencies NAUI and NASDS indicates that
Nitrox mixes are becoming a permanent part of the sport diving
community.

FUTURE
HISTORY

"What improvement may here after be made in diving I will not
pretend to say; yet I am convinced that there can be much progress in the art."
Johnny Green wrote these words in 1859. They are still valid!

Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety
Coordinator at the University of Michigan. He has authored more than 100 scuba
related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p.
54) is considered one of the best recreational sources of information In North
America.

Copyright 2001-2004 by Larry "Harris"
Taylor

All
rights reserved.

Use
of these articles for personal or organizational profit is specifically denied.